Update.prototype.canEdit = function() {
   return session.user == this.creator;
}

Update.prototype.constructor = function(date, text, creator) {
   if (date == null || text == null || creator == null) {
      throw "invalid parameters! date, text or creator are null!";
   }
   this.date = date;
   this.text = text;
   this.creator = creator;
}
